The human and financial impacts of osteoarthritis (OA) in the United States are immense. Regardless of the original cause, OA directly impacts the quality of life of nearly 21 million people in the US and is second only to cardiovascular disease in producing chronic disability [Altman and Moskowitz (1998) J Rheumatol 25, 2203-2212]. The world health organization estimates that OA accounts for 25% of visits to primary care physicians. The total cost of arthritis and other rheumatic conditions in 2003 were $128 billion—equivalent to 1.2% of the US gross domestic product [Morb Mortal Wkly Rep (2006) 55:1089-1092; Morb Mortal Wkly Rep (2007) 56:4-7]. Costs are rising substantially, with the estimated number of adults affected by OA reaching 59 million by 2020 [Lawrence, et al., Arthritis Rheum (1998) 41, 778-799]. Despite the tremendous burden of osteoarthritis in our society, it remains an incurable disease. There is currently no optimal solution for the clinical problem of the pre-arthritic condition of joint injury or degradation that will lead to future osteoarthritis. Treatments of osteoarthritis are designed to address the pain symptoms associated with OA, and no option offers treatment of the underlying pathology or improvements in the function of the deteriorating cartilage and bone.
Cartilage tissue engineering is a promising treatment for arthritic joints. Cartilage can be formed in-vitro from mesenchymal stem cells induced by the proper combination of growth factors. TGF-β1, which was originally identified as “cartilage-inducing factor” [Seyedin, et al., J Biol Chem (1986) 261, 5693-5695], is a potent inducer of cartilage formation in stem cells and is commonly used in cartilage tissue engineering. TGF-β1 stabilizes the chondrocyte phenotype, initially by promoting chondrogenesis in stem cells, and later by preventing hypertrophy. TGF-β1 also has potent anti-inflammatory effects in joints by counteracting the pro-inflammatory interleukin-1 pathways. Expression of TGF-β1 is reduced in OA, and local delivery of TGF-β1 is effective in stimulating cartilage repair in animal models of osteoarthritis and traumatic injury (reviewed in Grimaud, et al., Cytokine Growth Factor Rev (2002) 13, 241-257).
The Transforming Growth Factor beta (TGF-β1) family is a group of potent growth factors that increase chondrogenesis both in stem cells and in de-differentiated chondrocytes. The chondrogenic effects of TGF-β are widely used for tissue engineering of cartilaginous constructs. TGF-β activity is tightly regulated by many extracellular factors that control its bioavailability, most often through interfering with the binding of TGF-β1 to its receptors [Derynck and Miyazono, 2007. “The TGF-β family.” Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y].
Cartilage Oligomeric Matrix Protein (COMP) is an important cartilage protein that is essential for the structural integrity of the cartilage extracellular matrix. COMP binds to matrix components including collagens [Blumbach, et al., Matrix Biol (2008) 27, 306-318], aggrecan [Chen, et al., J Biol Chem (2007) 282, 24591-24598], matrilin [Mann, et al., J Biol Chem. (2004) 279(24):25294-8], fibronectin [Di Cesare, et al., Matrix Biol (2002) 21:461-470], and extracellular matrix protein-1 (ECM-1) [Kong, et al, Matrix Biol (2010) 29, 276-286]. Mature cartilage oligomeric matrix protein (COMP) is a large (524-kDa) disulfide-bonded homo-pentameric glycoprotein located mainly in the extracellular matrix of joint tissues such as cartilage, ligament, tendon and synovium (Hedbom, et al., (1992) J Biol Chem 267, 6132-6136; Newton, et al., (1994) Genomics 24, 435-439; Oldberg, et al., (1992) J Biol Chem 267, 22346-22350; and Morgelin, et al., (1992) J Biol Chem 267, 6137-6141). Each COMP monomer consists of an N-terminal coiled-coil domain, 4 EGF repeats, 8 TSP-3 repeats, and a thrombospondin C-terminal domain. The N-terminal coiled coil domain is responsible for forming the pentameric mature COMP protein, and contains two cysteine residues that covalently link adjacent chains (Efimov, et al., (1996) Proteins 24, 259-262; Malashkevich, et al., (1996) Science 274, 761-765; and Malashkevich, et al., (1996) Science 274, 761-765). The hydrophobic core formed by the coiled coil domains in COMP pentamers can bind small molecules such as retinol and vitamin-D (Ozbek, et al., (2002) EMBO J 21, 5960-5968). COMP has 4 EGF repeats, 13 TSP type 3 repeats, and a thrombospondin C-terminal domain (CTD), which together are responsible for binding interactions with other proteins and extracellular matrix components (Tan, et al., (2009) FASEB J 23, 2490-2501). Calcium-binding sites are located throughout COMP. Two of the four EGF repeats have calcium-binding sites, as do all of the TSP-3 repeats and the CTD. Calcium, and presumably other cations as well, alters the conformation of COMP and affects the binding interactions between COMP and other proteins.
COMP interacts with multiple other cartilage matrix components, including collagens type I, II and IX (Blumbach, et al., (2008) Matrix Biol 27, 306-318), proteoglycans such as aggrecan (Chen, et al., (2007) J Biol Chem 282, 24591-24598), non-collagenous matrix proteins such as fibronectin (Di Cesare, et al., (2002) Matrix Biol 21, 461-470) and matrilins (Mann, et al., J Biol Chem. (2004) 279(24):25294-8), as well as to glycosaminoglycans and heparin (Chen, et al., (2007) J Biol Chem 282, 24591-24598; DiCesare, et al., (1994) Eur J Biochem 223, 927-937). The repeated modular structure of COMP is critically important for its function as a ‘bridge’ that assembles multiple extracellular matrix components (Tan, et al., (2009) FASEB J 23, 2490-2501). For example, the COMP pentamer (but not monomeric COMP) simultaneously binds several free collagen molecules to accelerate collagen fibrillogenesis (Budde, et al., (2005) Mol Cell Biol 25, 10465-10478). In addition to binding extracellular proteins, COMP also contains an integrin-binding RGD sequence (Tan, et al., (2009) FASEB J 23, 2490-2501; Chen, et al., (2005) J Biol Chem 280, 32655-32661; Wang, et al., (2010) Circ Res 106, 514-525), and it can interact with the thrombospondin receptor CD47 (Rock, et al., (2010) Mol Cell Biochem 338, 215-224). The full implications of these cell-surface interactions in cartilage formation have not been discovered.
The expression of COMP is detected early in chondrogenic differentiation during embryonic skeletogenesis (Franzen, et al., (1987) Differentiation 36, 199-210). In the adult, COMP continues to be expressed in joint tissues, primarily in cartilage but also at detectable levels in ligaments, tendons, and synovium (Koelling, et al., (2006) Arthritis Res Ther 8, R56). COMP is expressed in isolated chondrocytes, but it is rapidly down-regulated during monolayer expansion of chondrocytes. This down-regulation is reversible, as it is rapidly re-expressed during chondrogenic redifferentiation (Zaucke, et al., (2001) Biochem J 358, 17-24). Our recent results suggest that COMP expression is also an early marker of the stem cell commitment to the chondrogenic lineage (Li, et al., (2011) Osteoarthritis and cartilage/OARS, Osteoarthritis Research Society).
The biological processes during which COMP expression is induced, including in vitro chondrogenic differentiation of stem cells and redifferentiation of passaged chondrocytes, are all heavily dependent on TGF-β signaling (Joyce, et al., (1990) J Cell Biol 110, 2195-2207; and Kawamura, et al., (1988) Dev Biol 130, 435-442). This suggests that there may be an interaction between COMP and TGF-β. Indeed TGF-β upregulates COMP mRNA expression and protein production in many systems (Recklies, et al., (1998) Arthritis Rheum 41, 997-1006; Barry, et al., (2001) Exp Cell Res 268, 189-200). However, a direct interaction between COMP and TGF-β proteins has not yet been identified. The binding of COMP to cell surface proteins and transmembrane receptors raises the intriguing possibility that, if COMP were able to directly bind to growth factors, it could act as a scaffold and influence the presentation of the growth factors to the receptors. Due to its repeated modular structure, COMP may bind several growth factors and increase their local concentration at the cell surface to enhance signaling.